Electrical transport across Au/Nb:SrTiO3 Schottky interface with different Nb doping

We have investigated electron transport in Nb doped SrTiO$_3$ single crystals for two doping densities. We find that the resistivity and mobility are temperature dependent in both whereas the carrier concentration is almost temperature invariant. We rationalize this using the hydrogenic theory for shallow donors. Further, we probe electrical transport across Schottky interfaces of Au on TiO$_2$ terminated n-type SrTiO$_3$. Quantitative analysis of macroscopic I-V measurements reveal thermionic emission dominated transport for the low doped substrate whereas it deviates from such behavior for the high doped substrate. This work is relevant for designing devices to study electronic transport using oxide-semiconductors.Comment: 10 Pages, 3 Figure

Empirical Bayes Approach to Controlling Familywise Error: An Application to HIV Resistance Data

Statistical challenges arise in identifying meaningful patterns and structures from high dimensional genomic data sets. Relating HIV genotype (sequence of amino acids) to phenotypic resistance presents a typical problem. When the HIV virus is under antiretroviral drug pressure, unfavorable mutations of the target genes often lead to greatly increased resistance of the virus to drugs, including drugs the virus has not been exposed to. Identification of mutation combinations and their correlation to drug resistance is critical in guiding efficient prescription of HIV drugs. The identification of a subset of codons associated with drug resistance from a set of several hundreds of codons presents a multiple testing problem. Statistical issues arising from genomic data multiple testing procedures include the choice of the null test-statistic distribution used to define cut-offs. Controlling familywise error rate implies controlling the number of false positives among true nulls. Given the large number of hypotheses to be tested, the number of true nulls is unknown. We apply two multiple testing procedures (MTPs) controlling familywise error rate: an adhoc augmented-Bonferroni method and a Empirical Bayes procedure originally proposed in van der Laan, Birkner and Hubbard(2005). Using simulations, we demonstrate that the proposed MTPs are less conservative than the traditional methods such as Bonferroni and Holm\u27s procedures. We apply the methods to HIV resistance data where we wish to identify mutations in the protease gene associated with Amprenavir resistance

High magnetoresistance at room temperature in p-i-n graphene nanoribbons due to band-to-band tunneling effects

A large magnetoresistance effect is obtained at room-temperature by using p-i-n armchair-graphene-nanoribbon (GNR) heterostructures. The key advantage is the virtual elimination of thermal currents due to the presence of band gaps in the contacts. The current at B=0T is greatly decreased while the current at B>0T is relatively large due to the band-to-band tunneling effects, resulting in a high magnetoresistance ratio, even at room-temperature. Moreover, we explore the effects of edge-roughness, length, and width of GNR channels on device performance. An increase in edge-roughness and channel length enhances the magnetoresistance ratio while increased channel width can reduce the operating bias.Comment: http://dx.doi.org/10.1063/1.362445

Arabidopsis accelerated cell death 11, ACD11, is a ceramide-1-phosphate transfer protein and intermediary regulator of phytoceramide levels

The accelerated cell death 11 (acd11) mutant of Arabidopsis provides a genetic model for studying immune response activation and localized cellular suicide that halt pathogen spread during infection in plants. Here, we elucidate ACD11 structure and function and show that acd11 disruption dramatically alters the in vivo balance of sphingolipid mediators that regulate eukaryotic-programmed cell death. In acd11 mutants, normally low ceramide-1- phosphate (C1P) levels become elevated, but the relatively abundant cell death inducer phytoceramide rises acutely. ACD11 exhibits selective intermembrane transfer of C1P and phyto-C1P. Crystal structures establish C1P binding via a surface-localized, phosphate headgroup recognition center connected to an interior hydrophobic pocket that adaptively ensheaths lipid chains via a cleft-like gating mechanism. Point mutation mapping con- firms functional involvement of binding site residues. A p helix (p bulge) near the lipid binding cleft distinguishes apo-ACD11 from other GLTP folds. The global two-layer, a-helically dominated, ‘‘sandwich’’ topology displaying C1P-selective binding identifies ACD11 as the plant prototype of a GLTP fold subfamily

Efficacy Studies of Malaria Treatments in Africa: Efficient Estimation with Missing Indicators of Failure

Efficacy studies of malaria treatments can be plagued by indeterminate outcomes for some patients. The study motivating this paper defines the outcome of interest (treatment failure) as recrudescence and for some subjects, it is unclear whether a recurrence of malaria is due to that or new infection. This results in a specific kind of missing data. The effect of missing data in causal inference problems is widely recognized. Methods that adjust for possible bias from missing data include a variety of imputation procedures (extreme case analysis, hot-deck, single and multiple imputation), inverse weighting methods, and likelihood based methods (data augmentation, EM procedures and their extensions). In this article, we focus on multiple imputation, two inverse weighting procedures (the inverse probability of censoring weighted (IPCW) and the doubly robust (DR) estimators), and a likelihood based methodology (G-computation), comparing the methods\u27 applicability to the efficient estimation of malaria treatments effects. We present results from a simulation study as well as results from a data analysis of malaria efficacy studies from Uganda

Selective Dynamic Nuclear Spin Polarization in Spin-Blocked Double-Dot

We study the mechanism of dynamical nuclear spin polarization by hyperfine interaction in spin-blocked double quantum dot system. We calculate the hyperfine transition rates and solve the master equations for the nuclear spins. Specifically, we incorporate the effects of the nuclear quadrupole coupling due to the doping-induced local lattice distortion and strain. Our results show that nuclear quadrupole coupling induced by the 5% indium substitution can be used to explain the recent experimental observation of missing arsenic NMR signal in the spin-blocked double dots.Comment: 4 pages, 3 figure

Absence of an intrinsic value for the surface recombination velocity in doped semiconductors

A self-consistent expression for the surface recombination velocity $S$ and the surface Fermi level unpinning energy as a function of light excitation power ($P$) is presented for n- and p-type semiconductors doped above the 10$^{16}$ cm$^{-3}$ range. Measurements of $S$ on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface $S$ is described by a power law in $P$ whereas for a passivated surface $S^{-1}$ varies logarithmically with $P$. Furthermore, the variation in $S$ with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that $S$ depends on $P$ throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of $S$ on a range of semiconducting materials are thus only valid for a specific excitation power.Comment: 10 pages, 7 figure

Electric field induced charge injection or exhaustion in organic thin film transistor

The conductivity of organic semiconductors is measured {\it in-situ} and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. The depletion layer thickness can be directly determined as a shift of the threshold thickness at which electric current began to flow. The {\it in-situ} and continuous measurement can also determine qualitatively the accumulation layer thickness together with the distribution function of injected carriers. The accumulation layer thickness is a few mono layers, and it does not depend on gate voltages, rather depends on the chemical species.Comment: 4 figures, to be published in Phys. Rev.

Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples